The applicant proposes to use an updated version of Holmberg's (1996) methodology to describe the status of the HIV epidemic in 96 large metropolitan areas. Annual survey data on syringe exchanges, SAMHSA data on drug treatment, and data on outreach programs to high-risk drug users (from a variety of sources) will be supplemented with Surveys of Community Experts in the first and last years of the project to provide up-to-date information on "response programs." Analyses will determine what characteristics of metropolitan areas are associated with (a) HIV prevalence and incidence among IDUs; (b) changes in HIV prevalence and incidence among IDUs; (c) responses to the epidemic; and (d) the proportion of IDUs in the population. Independent variables describing metropolitan areas, including distance from epidemic epicenters, local government budgets, laws about syringe access, population and economic characteristics, and many other variables will be obtained from public and private databases for periods (when possible) from 1980 to the present. Analyses will include both exploratory studies and, to the extent current science allows, will test relevant hypotheses about how distance from epicenters, policy environments, and population characteristics affect HIV spread among IDUs; and about how levels of need and degrees of specialized and general resources affect local responses. The proposed study will for the first time make data on local HIV epidemics among IDUs on changes in these local epidemics, and on response programs in the largest US metropolitan areas available for policy and research use. Analyses of these data will allow the investigator to explore, and test hypotheses about what social, economic, and policy contexts (a) affect vulnerability to HIV and other infections and (b) affect how communities respond to epidemic threats.